Recently, HCCI (Homogeneous-Charge Compression Ignition) combustion in which gasoline fuel mixed with air is combusted by self-ignition inside a sufficiently compressed cylinder has attracted attention. The HCCI combustion is a mode in which the mixture gas combusts at a plurality of positions simultaneously, and thus has a higher combustion speed of the mixture gas than in SI combustion (spark-ignition combustion) which is adopted for general gasoline engines. Therefore, the HCCI combustion is said to be significantly advantageous in terms of thermal efficiency. However, the HCCI combustion has issues such as a combustion start timing of the mixture gas (a timing that the mixture gas self-ignites) greatly varying due to an external factor (e.g., atmospheric temperature) and a control during a transient operation in which an engine load sharply changes is difficult.
Therefore, instead of combusting the entire mixture gas by self-ignition, it is proposed to combust a portion of the mixture gas by spark-ignition using a spark plug. That is, after forcibly combusting a portion of the mixture gas through flame propagation caused by spark-ignition (SI combustion), the remaining mixture gas is combusted by self-ignition (CI combustion). Hereinafter, such combustion is referred to as “partial compression-ignition combustion.”
For example, JP2009-108778A discloses an engine adopting a similar concept to the partial compression-ignition combustion. This engine causes flame propagation combustion by spark-igniting a stratified mixture gas which is formed around a spark plug by a supplementary fuel injection, and then performs a main fuel injection inside a cylinder warmed up by an effect of the flame propagation combustion, so as to combust through self-ignition the fuel injected in the main fuel injection.
Although the engine of JP2009-108778A can stimulate the CI combustion by the spark-ignition using the spark plug, a timing of the CI combustion actually starting (a delay time until self-ignition) is considered to vary to some extent due to an environment inside the cylinder. If the state where the start timing of the CI combustion varies from a target timing continues, loud combustion noise may occur and marketability of the engine may degrade. For example, when the cylinder is in an environment in which self-ignition occurs more easily than expected, the timing of self-ignition advances excessively and rising of an in-cylinder pressure becomes sharp, which may lead to loud combustion noise.